1. Field of Invention
The present invention relates to technology for preventing a rise in the power supply voltage in a drive circuit for driving a motor.
2. Description of Related Art
Pulse width modulation (PWM) drive methods that control energizing the motor coil by controlling the on/off state of specific switching devices in the motor drive circuit are commonly used to drive brushless motors in AV equipment. U.S. Pat. No. 5,309,078 (corresponding to Japanese Unexamined Patent Appl. Pub. H5-211780, and E.P. Publication No. 525,999), for example, teaches a widely used synchronous rectifier PWM drive technology for achieving low loss and high efficiency PWM drive.
The PWM drive technology taught in U.S. Pat. No. 5,309,078 is described more fully below with reference to FIG. 15. FIG. 15 shows part of a three-phase motor system that is driven by a drive unit 305 composed of a three-phase bridge.
The voltage detected by detection resistance 324 and torque control signal VREF are input respectively to the inverted Input node and the non-inverted input node of comparator 335. The torque control signal VREF sets the motor torque. The output node of the comparator 335 is connected to flip-flop circuit 336, and the inverted output of the flip-flop circuit 336 is input to two-phase non-superimposed clock generating unit 368. The two-phase non-superimposed clock generating unit 366 generates a pulse pair with a specific timing offset between the rising and falling edges in order to prevent current shoot-through in the drive unit 305. The output of the two-phase non-superimposed clock generating unit 368 is input to the energizing control unit 339 and synchronous rectifier control unit 340. The energizing control unit 339 supplies the drive signals that control the switching devices 325A, 325B, 325C to the high potential switching devices 325A, 325B, 325C, and the synchronous rectifier control unit 340 supplies the drive signals that control the low potential switching devices 326A, 326B, 326C to the low potential switching devices 326A, 326B, 326C.
Operation of this motor drive device is described next. The urging period is the period when drive power is supplied from the power source VM to the motor coils 310, 311, 312 by controlling the on state of the source phase (the phase in which motor current flows to the motor coil) high potential switching devices and the sink phase (the phase in which motor current flows from the motor coil) low potential switching devices.
The regeneration period is the period in which the motor current flowing to the motor coil circulates through the source phase low potential side switching device or the diode parallel connected to the source phase low potential side switching device, and a sink phase low potential side switching device. The drive unit 305 controls energizing the motor coils 310, 311, 312 through one cycle including an urging period and a regeneration period. The urging period and regeneration period include the offset timing generated by the two-phase non-superimposed clock generating unit.
For example, during the urging period node A is driven HIGH by high potential switching device 325A, node B is driven LOW by low potential switching device 326B, and node C is held in a high impedance state with both switching devices 325C and 326C off.
The motor current flowing through motor coils 310 and 311 between nodes A and B is converted to voltage by detection resistance 324. This voltage is compared by the comparator 335 with torque control signal VREF. If the voltage is greater than torque control signal VREF, the output of comparator 335 sets the flip-flop circuit 336 from which the inverted output is input to the two-phase non-superimposed clock generating unit 368. As a result, the output of the two-phase non-superimposed clock generating unit 368 is inverted, the energizing control unit 339 turns the switching device 325A on the high potential side of node A off, and synchronous rectifier control unit 340 turns the switching device 326A on the low potential side of node A on. At the same time the discharge circuit 341 releases switch 342 from specific voltage VS to start discharging. The discharge circuit 341 produces a time delay during which the high potential switching devices 325A, 325B, 325C are held off. When the capacitance voltage of the discharge circuit 341 becomes less than torque control signal VREF, the comparator 343 resets the flip-flop circuit 336, and again turns on the high potential switching device corresponding to the phase being driven.
As described above, if the low potential switching device 326A turns on when the high potential switching device 325A is off, the drive unit 305, and more particularly the motor coils 310 and 311, are shorted by two resistances, specifically the on resistors of low potential switching devices 326A and 326B. The motor current flows through a motor current path including motor coils 310 and 311 and switching devices 326A and 326B without passing any diodes. The current path carrying the motor current through motor coils 310 and 311 can therefore reduce the voltage drop on the current path more than is possible with current regeneration using diodes. Low loss and high efficiency can thus be achieved. This method of turning one switching device of a predetermined phase on during the regeneration period in which another switching device in the same predetermined phase is off is referred to as synchronous rectifier control. The period in which synchronous rectifier control is applied is called the synchronous rectifier period.
Some problems with this related art are described below. More particularly, some problems with the current-controlled PWM drive method taught in U.S. Pat. No. 5,309,078 are described below with reference to FIG. 16 and FIG. 17. FIG. 16 shows the phase A portion of the drive unit 305, the phase A motor coil 310, and the detection resistance 324 shown in FIG. 15. Reference EA denotes a back electromotive forceback electromotive force produced in the phase A motor coil proportionally to the rotational speed of the motor.
What happens when node A in FIG. 15 is driven HIGH by high potential switching device 325A, node B is driven low by low potential switching device 326B, switching devices 325C and 326C are off and node C is in a high impedance state, and torque control signal VREF is changed from a relatively high level (where back electromotive force EA is relatively high) to an extremely low level is considered below.
FIG. 17 is a timing chart describing the operation shown in FIG. 16. Periods T1 and T4 in FIG. 17 are the urging period in which drive power is supplied from the power source VM to the motor coil 310 through the phase A high potential switching device 325A, and periods T2 and T3 are the synchronous rectifier period in which motor current flows through the phase A low potential switching device 326A.
If torque control signal VREF falls sharply due to a reduce speed command, motor current IA1, which flows through phase A when the phase A high potential switching device is on, goes in a short time (period T1 in FIG. 17) to the maximum current IP level allowed by torque control signal VREF. As a result, the phase A high potential switching device goes off and the phase A low potential switching device goes on in a synchronous rectifier state, and motor current starts flowing as denoted by IA2 (in period T2 in FIG. 17). However, because the maximum current IP is low and the back electromotive force EA is high, the effect of the back electromotive force EA causes the motor current to start flowing in the opposite direction, that is, in the direction of IA3 (period T3 in FIG. 17). The back electromotive force EA also causes the motor current IA3 to rise during the synchronous rectifier period, which is longer than the urging period due to the reduce speed command. As a result, when the synchronous rectifier period ends, that is, when the phase A high potential switching device is on and the phase A low potential switching device is off, motor current IA4 flows back to the power source and causes the power supply voltage to rise (period T4 in FIG. 17).
While not shown in FIG. 17 for brevity, motor current flows through diode 327A back to the power supply during the shoot-through prevention period after the synchronous rectifier period ends, that is, when both high and low potential side switching devices for phase A are off.
A problem with the regeneration phase of the related art is that because motor current flows back to the power source when the synchronous rectifier period ends, the power supply voltage rises and can lead to device damage. Reducing device size and cost is also difficult with the related art because a capacitance to improve the current sink capacity of the power supply, a zener diode for voltage clamping, or some other external protection device is required to prevent a rise in the power supply voltage.
The problem of the power supply voltage rising after the synchronous rectifier period due to motor current flowing in the opposite direction as during the urging period is not limited to this method of PWM drive based on controlling the peak motor current, and the same problem is caused by speed reducing torque control and load changes in any motor drive method that uses PWM drive with synchronous rectifier control. In so-called voltage-controlled PWM drive methods, for example, that are based on comparing a carrier wave having a sinusoidal, trapezoidal, or similar waveform with a modulation wave having a triangular wave, sawtooth wave, or similar waveform, the power supply voltage also rises at the end of the synchronous rectifier period because speed reducing torque commands and load variations also cause the PWM duty ratio to drop and motor current to flow during the synchronous rectifier period in the opposite direction as during the urging period.